1. Field of the Invention
The present invention relates to an image sensing apparatus and a magnification chromatic aberration correction method.
2. Description of the Related Art
Newer digital still cameras have seen an increase in the number of pixels used, with cameras now commercialized that have mega-pixel image sensors of 10 million or more pixels. In general, in a digital still camera equipped with a mega-pixel image sensor, operation of the image sensor during electronic viewfinder (EVF) display and during moving image sensing is such that, in order to increase the display rate, a driving method is adopted that thins the lines so that they match the number of display lines of the liquid crystal display apparatus. In addition, in order to obtain a sufficient value of pixel data when sensing during short exposure flashes, there is also a driving method that adds together adjacent pixels and reads them out.
One of the aberrations generated in the optical systems in these cameras (image sensing apparatuses) is magnification chromatic aberration. Chromatic aberration is caused by differences in refractive index due to wavelength, and since focal length differs with color with magnification chromatic aberration in particular, is a phenomenon in which the position of the image on the image plane (that is, the magnification) appears to shift with each color. Magnification chromatic aberration can be corrected to some extent by the material of the lens. However, in that case, because the material becomes expensive and so forth, the cost of manufacturing the lens also increases. For this reason, a number of methods that correct the magnification chromatic aberration using signal processing have been proposed.
For example, according to the art disclosed in Japanese Patent Laid-Open No. 11-161773, first, magnification chromatic aberration data for the lens for each color is written to a data storage area, and magnification correction by enlarging and reducing the image based on the magnification chromatic aberration data is carried out each time an image is obtained through that lens. Then, after magnification correction, the images of each color are synthesized into a single image, accomplishing magnification chromatic aberration correction.
In addition, in Japanese Patent Laid-Open No. 2001-186533, a technique of carrying out magnification chromatic aberration correction in an image sensor that receives light of three colors such as RGB at a single image sensor plane is disclosed. In this method, synchronization and magnification chromatic aberration correction of the image is carried out by obtaining an interpolation coefficient based on chromatic aberration correction information when interpolating missing color signals for the pixels for reproduction as image data.
However, Japanese Patent Laid-Open No. 11-161773 and Japanese Patent Laid-Open No. 2001-186533 do not take into consideration magnification chromatic aberration correction in a case in which the image sensor is read out in different modes, such as a thinning readout mode and a pixel addition mode.
FIGS. 4A through 4C are diagrams illustrating a pixel arrangement in a case in which image data is read out in a thinning readout mode during EVF display and moving image sensing. FIG. 4A shows the pixel arrangement in the image sensor, FIG. 4B shows the pixel arrangement on the image sensor of image data read out in the thinning readout mode, and FIG. 4C shows the pixel arrangement in a state in which the image data read out in the thinning readout mode is stored in a memory or the like. As can be seen from FIGS. 4A-4C, the pixel arrangement of image data read out in the thinning readout mode (FIG. 4C) is different from the pixel arrangement in the image sensor (FIG. 4B). As a result, if magnification chromatic aberration correction is carried out on the image data that is read out in the thinning readout mode without taking into consideration this difference in position of the pixel arrangements, conversely the image quality deteriorates.
FIGS. 14A through 14C are diagrams illustrating pixel arrangements in a case in which image data is thinned and read out in the pixel addition mode. As can be seen from FIG. 11, the pixel arrangement of image data read out in the pixel addition mode (FIG. 14C), when compared to the pixel arrangement of the image sensor (FIG. 14A), not only differs from the thinned and read-out pixel arrangement but also shows a shift in the center of gravity due to addition (see FIG. 14B). As a result, if magnification chromatic aberration correction is carried out on the image data that is read out in the pixel addition mode without taking into consideration this difference in position of the pixel arrangements, the image quality deteriorates due to correction.